This invention is in the field of antennas and more specifically relates to antennas of portable radio equipment. The invention further relates to portable electronic equipment such as a radio transmitter/receiver, fitted with said antennas.
Antennas of portable radio-frequency (RF) equipment may in use inadvertently impinge upon an external high voltage source such as an overhead power line. In the absence of any means of protection, contact of the antenna with the voltage source poses a serious hazard to the RF equipment and its carrier. If the carrier is a vehicle, it may be seriously damaged by the resulting high voltage electric current passing through it. For a human carrier, the electric current may be fatal. Antennas of portable RF equipment are therefore commonly provided with means to protect the carrier from electric shock. In the case of a human carrier, the electric current must be lowered to below the so called let-go level which is about 10 mA for males and 7 mA for females.
One method of protection is provided by coating the antenna with an insulating compound. This, however, adds considerable weight to the antenna and does not provide reliable protection as the coating eventually becomes scratched and attenuated due to abrasion of the antenna with external objects.
Antenna protection is also provided by a series-opening switch. However, the long opening time of the switch, exceeding several milliseconds, compromises the protection afforded. Opening switches are also very bulky and are thus not suitable for foot-carried RF equipment. Another method of antenna protection is bypassing high voltage shocks to ground using closing switches. This method however is not applicable to foot carried RF equipment. Moreover, closing and opening switches are activated by a sensor that inevitably introduces a dangerous delay in their operation and may, conversely, also produce false alarms.
Another method of antenna protection involves inserting a high-pass filter into the antenna between the RF equipment and carrier on the one hand and the distal section of the antenna likely to contact an overhead power line on the other, as disclosed, for example in U.S. Pat. No. 4,513,338. This patent discloses an antenna protection device comprising a linear capacitor having a Teflon(trademark) dielectric to suppress the low-frequency current component while causing minimal signal loss at the higher operating frequencies. In practice, however, with a linear filter such as this, the low frequency component cannot be filtered out below the let-go level without an unacceptable loss in the intensity of the high frequency signal.
The present invention provides a device for use with portable RF equipment coupled to an antenna for protecting the equipment and its carrier from electric shocks resulting from antenna contact with an external power source such as overhead power lines. The invention is designed to be incorporated into an antenna and is easily retrofitted into existing antennas. The invention causes minimal signal loss even at relatively low signal frequencies of several MHz while suppressing the line frequencies of 50-60 Hz to below the let-go level.
The device of the invention comprises one or more non-linear high voltage capacitors. Capacitors having these characteristics are known in the art and may be purchased for example from Thomson-CSF, Vishay, Ceramite, or Morgan-Matroc. These capacitors have ceramic dielectrics generally known in the art as Class II, or Type II ceramics that are generally strontium-based and have a negative DC voltage coefficient. In accordance with the invention it was found that at line frequencies these capacitors also have a negative AC voltage coefficient. This non-linearity may reach 90-95%, in which case at line frequency the capacitor possess 10-20 fold greater impedance compared to a linear capacitor with the same nominal capacitance. Such a capacitor will reduce a 50-60 Hz line current component 10-20 fold with minimal effect on a signal component of over 15 MHz.
The present invention thus provides a device for insertion between first and second antenna sections and capable of significantly cutting the passage of an alternating electric current having a frequency lower than a first desired predetermined value, characterized in that it comprises a non-linear capacitor with first and second capacitor plates linked to first and second terminal means and serving for the connection of said first and second antenna sections, which non-linear capacitor is characterized by a negative low-frequency voltage coefficient, whereby the passage of an alternating electric current of a frequency higher than a second desired predetermined value remains essentially unaffected.
The invention further provides an antenna assembly having at least two antenna sections connected to the first and second terminals of a protection device of the kink specified.
Still further, the invention provides radio equipment fitted with such an antenna assembly.